Figure 1.

When a molecule extends across the periodic boundary atoms that belong to the same molecule interact through different periodic copies. When computing the molecular virial, atoms i₁, j₁, k₁ are associated with the molecule centered in $\overrightarrow{r}$₁, while the atoms l₂, m₂, n₂ are associated with a copy that is positioned at $\overrightarrow{r}$₂ = $\overrightarrow{r}$₁ + $\overrightarrow{\delta }$. Therefore, in implementing the molecular virial, it is necessary to allocate multiple molecular coordinates to a single molecule.